Reactive molecular metal catalysts are an important class of molecules because they permit chemical transformations not otherwise possible by creating a particular environment around a reactive metal center. This is made possible by using ligands, which provide certain steric and electronic properties at the metal center.
One common technique for creating reactive molecular metal catalysts is to control the steric bulk of the ligand on the metal center. The addition of large functional groups to a ligand can influence reaction rate, coordination number, substrate reactivity, and kinetically stabilize reactive intermediates. Controlling the steric bulk of the ligand has been applied to many different ligand types.
The β-diketonate backbone can be substituted with functional groups in three positions. The most common β-diketone ligand, acetylacetonone, imparts very little steric influence on the resulting metal complex, often forming M(acac)3 or multi-metallic aggregates in solution and solid states.
The acetylacetonate (“acac”) ligand is probably the most used β-diketonate. Such acac ligands are among the most pervasive classes of ligands found in inorganic and organometallic chemistry. They have been used and investigated for over a century.
Acetylacetonate (“acac”) ligands have been used to coordinate virtually every transition, main group, and f-block element. Metal-acac complexes are commercially available for every transition metal except Tc, W, Re, Os, and Hg and group 13 element (except boron). These complexes commercially available catalysts for polymerization, hydrogenation, oxidation, and condensation reactions, and used as molecular precursors for nanoparticle and thin film materials.
Although this ligand class has been explored for over a century, the community has never developed an effective way of introducing large steric bulk onto β-diketonate (acac) ligands. Accordingly, the molecular metal catalyst community has not benefited from molecular metal catalysts supported by sterically bulky β-diketonate (acac) ligands.